Effects of compressibility and transition to turbulence on flow through microchannels

A detailed experimental study of flow through long microchannels of hydraulic diameter ranging from 60.5 to 211 μm has been carried out. The internal pressure distribution along the length of the channel has been measured to analyze the local flow behaviour. The effects of compressibility and transition to turbulence occurring in the microchannel flow were investigated in detail. In addition, the resulting flow has been analyzed numerically using a commercially available CFD code, FLUENT. It has been shown that there are no special micro-scale effects, including early transition to turbulence at least in the present range of hydraulic diameters after the significant effects of compressibility are accounted appropriately.     

Characterization of electroosmotic flow in rectangular microchannels

In this paper, the electroosmotic displacing process between two solutions (namely the same electrolyte of different concentrations) in a rectangular microchannel is studied theoretically and experimentally. Firstly, the electric potential and velocity field in a rectangular microchannel are obtained by solving the governing equations. Fourier transform method is used to solve the electrolyte concentration profile equation. The electric current versus time curve through the microchannel is predicted based on the concentration profile obtained. The current monitoring technique is then used to study the electroosmotic displacing process. The results from the measured current–time relations agree well with those from the prediction, suggesting a reliable theoretical model developed in this study.     

Numerical study of condensation flow regime transition in wavy microchannels

A numerical research on flow regime transition in wavy microchannels was conducted. The model was based on the volume of fluid approach and user-defined routines including interfacial mass transfer and latent heat. The observed droplet flow, annular–wavy flow, injection flow, and slug–bubbly flow were qualitatively compared against experimental data and transition lines were established. The effects of inlet vapor velocity, wall heat flux, and microchannel geometry characteristics on the annular length, occurrence frequency of injection flow, initial slug volume, and bubble detachment frequency were investigated.     

Large-eddy simulation of transition to turbulence in natural convection in a horizontal annular cavity

Large-eddy simulations (LES) of transition to turbulence in a horizontal annular cavity are performed, using a dynamic sub-grid scale model and second order schemes for time and space discretizations. Solutions for Prandtl number of 0.707 and Rayleigh number up to 7.5 × 10⁵ are obtained. The onset of transition to turbulence and turbulence regimes are pointed out, as well as the dynamic characteristics of the thermal plume transition. The instantaneous and time average behavior of the flows, related to the velocity and temperature fields, are analyzed and compared with numerical and experimental results from other authors. The influence of transitional and turbulent flows on local and mean Nusselt number are also investigated.     

Characterization of frictional pressure drop for liquid flows through microchannels

This paper investigates pressure driven liquid flow through round and square microchannels fabricated from fused silica and stainless steel. Pressure drop data are used to characterize the friction factor for channel diameters in the range 15-150 μm and over a Reynolds number range 8-2300. Distilled water, methanol, and isopropanol were used in this study based on their distinct polarity and viscosity properties. Distinguishable deviation from Stokes flow theory was not observed for any channel cross-section, diameter, material, or fluid explored.     

Onset of transition to turbulence in natural convection with gas along a vertical isotherm plane

It is generally accepted that the onset of transition to the turbulence in free convection along a vertical isotherm plane with gas occurs at a given Grashof number. A more subtle reality became apparent thirty years ago in the course of experiments carried out with either small installations and pressurized gas or under atmospheric pressure with an experimental installation of great height. To elucidate the problem, a re-examination of data set and a new analysis have been made. This has led to a physical interpretation of what was observed : the origin of the instability which is triggered off in this boundary layer lies in interaction between the zone beyond this boundary layer where there is a vertical stratification of temperature and the boundary layer near the plane. This is based on the comparison between characteristic times calculated for the two zones in question.     

Experimental investigation of the Reynolds criterion for transition from the transition zone to complete turbulence in 3D internally finned tubes

The relations between the point for transition from the transition zone to complete turbulence and the fin height, fin width as well as fin density in 3D internally finned tubes have been investigated experimentally. At the same time, the performance of heat transfer and pressure drop in the tubes have been investigated when the flow is completely turbulent. Three expressions hale been obtained for (1) the Reynolds criterion for transition from the transition zone to complete turbulence, (2) the Nusselt number of complete turbulence in the tubes, and (3) the Fanning friction factor of complete turbulence in the tubes. These expressions can be used in engineering designs. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 183–188, 1999     

Bubble dynamics in microchannels. Part II: two parallel microchannels

The present work investigates experimentally the bubble dynamics in two parallel trapezoidal microchannels with a hydraulic diameter of 47.7 μm for both channels. The fabrication process of the two parallel microchannels employs a silicon bulk micromachining and anodic bounding process. The results of this study demonstrate that the bubble growth and departure is generally similar to that in a single microchannel, i.e., bubbles, in general, grow linearly with time and their departure is governed by surface tension and drag due to bulk two-phase flow. For the two low mass flow rates, the growth of bubble in slug flow is also investigated. It is found that the bubble grows in the axial direction both forward and backward with its length increases exponentially due to evaporation of the thin liquid film between the bubble and heating wall. However, the coefficient of exponent is much smaller than that caused by evaporation due to the limitation effect of liquid pressure around the bubble.     

Boiling incipience in microchannels

The available data dealing with boiling incipience of water in microtubes (tubes with diameters in the 0.1-1 mm range) are analyzed. Macroscale models and correlations appear to under predict the heat fluxes that lead to the boiling incipience in microtubes. It is suggested that boiling incipience in microchannels may be controlled by the thermocapillary force that tends to suppress the microbubbles that form on wall cavities. Accordingly, a semi-empirical method is proposed for predicting the boiling incipience in microtubes. The effect of the turbulence characteristics of the microtube on the proposed method is also examined.     

Contribution of homogeneous reactions to hydrogen oxidation in catalytic microchannels

Combustion of lean H₂/air mixtures in Pt-coated microchannels is investigated numerically in planar geometry. Examining the relative importance of hydrogen oxidation in the bulk gas as compared to surface reactions under different operating conditions is the main focus of the present work. A collocated finite-volume method is used to solve the governing equations. Detailed gas phase and surface reaction mechanisms along with a multi-component species diffusion model are used. In microchannels, due to effective heat and radical losses to the walls, the combustion characteristics are greatly influenced as the channel size is reduced. While catalytic walls help in sustaining gas phase reactions in very small channels by reducing heat losses to the walls owing to exothermic surface reactions, they also inhibit homogeneous reactions by extracting radicals due to typically high absorption rates of such species at the walls. Thus, the radical chain mechanism can be significantly altered by the presence of wall reactions, and the build-up of radical pools in the gas phase can be suppressed as a consequence. In the present work the effects of three key parameters, the channel height, the inlet mass flux and the equivalence ratio of the inlet mixture on the interaction between the gas phase and surface reactions are explored. In each case, the limiting values beyond which the gas-phase reactions become negligible compared to surface reactions are identified for lean hydrogen/air mixtures.     

Characterization of a porous medium employing numerical tools: Permeability and pressure-drop from Darcy to turbulence

A large set of microscopic flow simulations in the Representative Elementary Volume (REV) of a porous medium formed by staggered square cylinders is presented. For each Reynolds number selected, 10 different porosities are simulated in the 5–95% range. The Reynolds number is varied from Re = 10⁻³ to Re = 10⁵, covering the Stokes flow regime, the laminar flow regime and the turbulence flow regime. Low and moderate Reynolds number flow solutions (Re  200) are achieved by numerically solving the 2D Navier–Stokes equations. Reynolds Averaged Navier–Stokes equations are employed to simulate the turbulence regime. Numerical results allow the investigation of the microscopic features of the flow as a function of the porosity and Reynolds number. Based on these microscopic results, the permeability of the porous medium is computed and a porosity-dependent correlation is developed for this macroscopic parameter. The Darcy–Forchheimer term or, equivalently, the friction factor, is also computed to characterize the porous medium for the complete range of porosity and Reynolds number simulated. The Forchheimer coefficient is found to be weakly dependent on the Reynolds number and strongly dependent on the porosity if the flow is fully turbulent. A porosity-dependent correlation is proposed for this quantity for high Reynolds numbers.     

Heat transfer in rectangular microchannels

Convection heat transfer in a rectangular microchannel is investigated. The flow is assumed to be fully developed both thermally and hydrodynamically. The H2-type boundary condition, constant axial and peripheral heat flux, is applied at the walls of the channel. Since the velocity profile for a rectangular channel is not known under the slip flow conditions, the momentum equation is first solved for velocity. The resulting velocity profile is then substituted into the energy equation. The integral transform technique is applied twice, once for velocity and once for temperature. The results show a similar behavior to previous studies on circular microtubes. The values of the Nusselt number are given for varying aspect ratios.     

Slip flow in elliptic microchannels

Microscale fluid dynamics has received intensive interest due to the emergence of Micro-Electro-Mechanical Systems (MEMS) technology. When the mean free path of the gas is comparable to the channel's characteristic dimension, the continuum assumption is no longer valid and a velocity slip may occur at the duct walls. The elliptic cross-section is one useful channel shape that may be produced by microfabrication. The elliptic microchannels have potential practical applications in MEMS. Slip flow in elliptic microchannels has been examined and a detailed theoretical analysis has been performed. A solution is obtained using elliptic cylinder coordinates and the separation of variables method. A simple model is developed for predicting the Poiseuille number in elliptic microchannels for slip flow. The developed model may be used to predict mass flow rate and pressure distribution of slip flow in elliptic microchannels.     

Condensation of steam in silicon microchannels

A visualization study was performed on condensation of steam in microchannels etched in a 〈100〉 silicon wafer that was bonded by a thin Pyrex glass plate from the top. The microchannels had a trapezoidal cross section with a hydraulic diameter of 75 μm. Saturated steam flowed through these parallel microchannels, whose walls were cooled by natural convection of air at room temperature. The absolute pressure of saturated steam at the inlet ranged from 127.5 kPa to 225.5 kPa, and the outlet was at atmospheric pressure at approximately 101.3 kPa with the outlet temperature of the condensate ranging from 42.8 °C to 90 °C. Stable droplet condensation was observed near the inlet of the microchannel. When the condensation process progressed along the microchannels, droplets accumulated on the wall. As the vapor core entrained and pushed the droplets, it became an intermittent flow of vapor and condensate at downstream of the microchannels. The traditional annual flow, wavy flow and dispersed flow observed during condensation in macrochannels were not observed in the microchannels. Based on a modified classical droplet condensation theory, it is predicted that the droplet condensation heat flux increases as the diameter of the microchannel is decreased. It is also predicted that the droplet condensation heat flux of saturated steam at 225.5 kPa can reach as high as 1200 W/cm² at ΔT=10 °C in a microchannel having a hydraulic diameter of 75 μm.     

Effects of freestream turbulence on bypass transition of separated boundary layer on low-pressure turbine airfoils

This paper presents experimental studies on bypass transition of separated boundary layer on low-pressure turbine airfoils,focusing on the effects of freestream turbulence on the transition process.Hot-wire probe measurements are performed on the suction side of an airfoil in the low-pressure linear turbine cascade at several Reynolds number conditions.Freestream turbulence is enhanced by use of turbulence grid located upstream of the cascade.The results of this experimental study show that the location of boundary layer separation does not strongly de-pend on the freestream turbulence level.However,as the freestream turbulence level increases,the size of separa-tion bubble becomes small and the location of turbulent transition moves upstream.The size of separation bubble becomes small as the Reynolds number increases.At low freestream turbulence intensity,the velocity fluctuation due to Kelvin-Helmholtz instability is observed clearly in the shear layer of the separation bubble.At high frees-tream turbulence intensity,the streak structures appear upstream of the separation location,indicating bypass transition of attached boundary layer occurs at high Reynolds number.     

Non-linear analyses of flow boiling in microchannels

Recently, four unstable boiling cases with different fluctuating amplitudes were observed in parallel silicon microchannels having a hydraulic diameter of 186 μm. These were: the liquid/two-phase alternating flow (LTAF) at two different heat fluxes, the continuous two-phase flow (CTF) at medium heat flux and medium mass flux, and the liquid/two-phase/vapor alternating flow (LTVAF) at high heat flux and low mass flux. In this paper, data of these unstable boiling cases are analyzed using the following methods: correlation coefficient, attractor reconstruction, correlation dimension and largest Lyapunov exponent. The processes responsible for appearance of chaotic oscillations in microchannels, such as nucleation, stability of bubbly flow, vapour core stability and vapour-phase flow stability, are discussed. It is shown that under certain conditions, the microchannels system works as a thermal oscillator. It was found that heat supplied to the microchannels increases the heating surface temperature while the appearance of the two-phase flow inside the channels decreases the heating surface temperature. The mechanism involving an increase in heating surface temperature is supported by phenomena of blocking the liquid flow in microchannels by the two-phase flow.     

Flow boiling in microchannels and microgravity

A critical review of the state of the art of research on internal forced convection boiling in microchannels and in microgravity conditions is the main object of the present paper.     

Pressure-driven electrokinetic slip-flow in planar microchannels

This paper presents an analytical solution for pressure-driven electrokinetic flows in planar microchannels with velocity slip at the walls. The Navier–Stokes equations for an incompressible viscous fluid have been solved along with the Poisson–Boltzmann equation for the electric double layer. Analytical expressions for the velocity profile, average electrical conductivity, and induced voltage are presented without invoking the Debye–Hückel approximation. It is known that an increase in the zeta-potential leads to an increase in the flow-induced voltage; however, it is demonstrated that the induced voltage reaches a maximum value at a certain zeta-potential depending on the slip coefficient and the Debye–Hückel parameter, while decreasing rapidly at higher zeta-potentials. The present parametric study indicates that liquid slip at the walls can increase the maximum induced voltage very significantly.     

Forced convection in slightly curved microchannels

To address the effects of curvature, initial conditions and disturbances, a numerical study is made on the fully-developed bifurcation structure and stability of the forced convection in curved microchannels of square cross-section and curvature ratio 5 × 10⁻⁶. No matter how small it is, the channel curvature always generates the secondary flow in the channel cross-plane which increases the mean friction factor moderately and the Nusselt number significantly. Unknown initial conditions of convection lead to the co-existence of multiple steady fully-developed flows of various structures. Ten solution branches (either symmetric or asymmetric) are found with eight symmetry-breaking bifurcation points and thirty-one limit points. Thus a rich solution structure exists with the co-existence of various flow states over certain ranges of governing parameters. Dynamic responses of the multiple steady flows to finite random disturbances are examined by the direct transient computation. It is found that possible physically realizable fully-developed flows under the effect of unknown disturbances evolve, as the Dean number increases, from a stable steady 2-cell state at lower Dean number to a temporal periodic oscillation, another stable steady 2-cell state, a temporal intermittent oscillation, and a chaotic temporal oscillation. There exist no stable steady fully-developed flows in some ranges of governing parameters. Both the mean friction factor and the mean Nusselt number are also obtained and analyzed with their correlating relations listed.